Interlaboratory test for stable carbon isotope analysis of dissolved inorganic carbon in geothermal fluids

Abstract

Rationale:Stable carbon isotope ratios have many applications in natural sciences. In the first worldwide interlaboratory proficiency test, the discrepancies in measured δ 13 C DIC values of natural waters were up to σ = ±3‰. Therefore, we continued the investigation on the analytical data quality assurance of individual laboratories and internal consistency among laboratories worldwide. Methods:We designed and performed an interlaboratory comparison exercise for δ 13 C analyses of ten water and two solid samples (Na 2 CO 3 , CaCO 3 ), including two synthetic samples prepared by dissolving the carbonates individually. Three laboratories analyzed an additional sample set to assess solution stability, at least one month after the first set analysis period. The δ 13 C values were measured using dual inlet isotope ratio mass spectrometry (DI-IRMS) or continuous flow (CF)-IRMS. Results:The δ 13 C values of solid Na 2 CO 3 and its aqueous solution were −5.06 ± 0.21‰ and 5.32 ± 0.24‰, respectively, while the δ 13 C value of solid CaCO 3 was −4.49 ± 0.93‰. Similarly, the lake water has a consistent value (2.45 ± 0.19‰).The δ 13 C values of geothermal water have a wide dispersion among individual laboratory measurements and among those of different laboratories; however, a trend exists in the δ 13 C values measured at the three sampling points of each well. Conclusions:The δ 13 C values of solid Na 2 CO 3 and its solution, and lake water (i.e. DIC concentration samples >100 mg/L carbon) are consistent among all the participating laboratories. The dispersion in the δ 13 C values of solid CaCO 3 is associated with its lower chemical affinity than that of Na 2 CO 3 . The poor reproducibility in the δ 13 C values of geothermal fluids, collected at three points of a geothermal well, despite overall consistent trends regarding their collection points suggests inadequate sample handling (atmospheric CO 2 exchange) and/or inappropriate analytical approaches (incomplete H 3 PO 4 acid reaction).